This invention relates to a steel cord for reinforcing an automobile tire, a method of manufacturing the steel cord, and an automobile tire reinforced with the steel cord.
A bundle twisted steel cord is well-known as one example of the steel cord which is used to reinforce an automobile tire. A conventional method of manufacturing a steel cord with a brancher type wire twisting machine (cf. Japanese Patent Application (OPI) No. 92329/1976 (the term "OPI" as used herein means an "unexamined published application") will be described with reference to FIG. 1. Filaments passing through several guide holes in an end plate 1 are drawn together by a collector 2 and temporarily twisted by a first twisting roller 3. The filaments thus treated are then supplied to a guide 4 and a guide roller 6 at the end of a flyer 5, and then to a guide roller 6 at the end of another flyer 5 and a guide 4. That is, as the two flyers 5 rotate, the filaments are twisted between the guides 4 and 4, and are temporarily twisted by a second twisting roller 7 again. The filaments thus treated, namely, a steel cord A, is wound on a winding reel 8. The end plate 1, as shown in FIG. 4 has a central guide hole 1a and a number of peripheral guide holes 1b. A core strand formed by twisting three filaments is inserted into the central guide hole 1a. Side strands, namely, filaments are inserted into the peripheral guide holes 1b. Accordingly, the resultant steel cord, as shown in FIG. 5, comprises: the core strand Aa made up of three filaments (black circles); and several side strands (white circles) Ab twisted around the core strand Aa. That is, in the conventional bundle twisted steel filament, a number of filaments form at least two layers, and have the same twisting direction and the same twisting pitch, and the diameters of the coils formed by the filaments are constant respectively. Accordingly, the filaments forming the central layer at a point of the steel cord form the central layer at any other points of the steel cord, and similarly the filaments forming the intermediate layer at a point of the steel cord form the intermediate layer at any other points of the steel cord.
As is apparent from the above description, in the conventional bundle twisted steel cord, the filaments are arranged in such a manner that filaments forming a layer are never shifted to form other layers. Therefore, the filaments forming the intermediate layer and the central layer are liable to come off. During running of the automobile, the filaments of the layers other than the outermost layer of this conventional bundle twisted steel cord may come off inside the tire, thus causing the blowout of the tire or the leakage of air from the tire. Furthermore, sometimes the filaments come off outside the tire, as a result of which water goes into the tire, thus corroding the steel cord.